A pitch class is a set (equivalence class) of all pitches that are a whole number of octaves apart, e.g., the pitch class C consists of the Cs in all octaves. Thus the pitch class "C" is the set

${..., C_{-2}, C_{-1}, C_0, C_1, C_2 ...}$

In terms of frequencies expressed in hertz, assuming a base frequency for middle C of 262 Hz, this would be {... 65.5, 131, 262, 524, 1028 ...}. In terms of midi note numbers, we can write it as {... 36, 48, 60, 72, 84 ...}.

Links

The Wikipedia article on it is:

Pitch class